It's the temperature of the air that matters to a plant, not the wind chill. Calm air can get much colder.
Posted on January 21, 2014 by Mark Longstroth, Michigan State University Extension
We have had some cold winter weather this year with wind chills down to -20 or -30 degrees Fahrenheit and some school closings because of the snowy conditions and extreme cold. Meteorologists like to emphasis the precautions we should take in cold, windy weather and cite the wind chill to tell us to bundle up and protect exposed flesh.
Wind chill is a measure of what the combination of wind and temperature feels like. In calm conditions, there is a fine layer of air called the boundary layer that insulates us from the cold. As the wind blows, it blows away this boundary layer and the cold wind can carry away heat from our bodies faster because there is no air insulating us. The faster the wind blows the more heat it can carry away. Think of a cooling breeze on a hot summer day.
When people call and ask if it was cold enough to injure their plants, they often tell me what the wind chill was. I am more interested in the actual cold temperature. Wind chill really only matters if you are trying to stay warmer than the air temperature. Warm blooded animals like ourselves, our pets or livestock are trying to keep our bodies warm because if we get too cold we stop working.
Plants are usually close to air temperature. On a cold winter day, they are close to the temperature of the air. If the wind blows hard it cannot cool down the plant any colder than the air temperature. If the plant gets colder than the air temperature, the warmer air will warm the plant. On a sunny day, if the plant warms in the sun it may get much warmer than the air if the conditions are calm. If conditions are windy, then the plant will only get a little warmer than the air as heat is carried away more quickly.
Under real cold conditions when much of the water in a plant is frozen, a strong, dry wind will carry away moisture and dehydrate the plant. Desiccation is a bigger problem when temperatures are above freezing and it is windy for plants that retain their leaves or needles in the winter.
Under calm conditions it can get much colder after the sun goes down. Without the wind to stir up the air, cold air collects close to the surface and flows into cold areas. When we have snow cover, the snow reflects a lot of the sun’s heat back to the sky. After the sun goes down, the cold snow chills the air above it and without a wind to stir it up and mix with the warmer air above, a very cold layer develops just above the snow. Often we see the worst winter injury close to the ground just above the insulating snow. Generally I am happy when the wind is blowing in the winter because it prevents cold layers from forming close to the ground.
Many of the plants we grow here in Michigan can handle the cold conditions of a Michigan winter. People are always planting new varieties to see how they will do. With our frequent drops to zero and below in 2014, this will be a good test winter to determine what varieties should not be planted here in Michigan.
For more information, see the related Michigan State University Extension articles
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